vcs-svn: rename check_overflow arguments for clarity

vcs-svn: rename check_overflow arguments for clarity

Code using the argument names a and b just doesn't look right (not
sure why!).  Use more explicit names "offset" and "len" to make their
type and meaning clearer.

Also rename check_overflow() to check_offset_overflow() to clarify
that we are making sure that "len" bytes beyond "offset" still fits
the type to represent an offset.

Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>